/**
 * 计算9！的阶乘
 */
a = 4, b = 1;
function exec(a, b){
  if(a < 0){
    return 0;
  }else if(a===0){
    return 1
  }else if(a===1){
    return b;
  }else{
    return exec(a-1, a*b);
  }
}

console.log(exec(a,b))
